It is well known to represent an image digitally by dividing the image into a large number of segments or resolution elements, denoted pixels, and allocating digital values, denoted pixel values, to each pixel. Typically, the image is divided into a matrix of rows and columns of pixels and the pixel values are typically stored in an array in a digital memory.
For example, a grey tone image may be represented digitally by a digital image comprising pixels each of which has one pixel value representing the grey tone of the corresponding pixel. Similarly, a color image may be represented by a digital image comprising pixels each of which holds three pixel component values, e.g. a value for each of the colors red, green, and blue.
An image recording system, such as an electronic camera, a digital camera, an electronic scanner, a digital scanner, uses a solid state imaging device, typically a charge coupled device (CCD), for recording of an image.
The CCD is an array of a large number of light sensitive elements connected to each other forming an analogue shift register. In each detector of the CCD, a charge is formed that is proportional to the light energy incident on the detector during an integration period. After completion of the integration period, the analogue charge of each detector is shifted serially out of the CCD and is converted to a digital value whereby a digital representation of the recorded image is formed. Each pixel value of the digital image is equal to the digitized charge of the corresponding CCD element. The pixel values may be transferred to an external computer through a computer interface or may be stored in a memory, such as a memory card, or on a rotating magnetic recording medium, in the apparatus containing the CCD.
In the following, the term pixel may refer to the smallest graphical resolution element of an image as reproduced on a viewable medium, e.g. a computer screen, paper, etc, or a storage location in a memory holding the corresponding pixel component values, or a sensor element in a sensor array. The actual meaning of the term pixel will be apparent from the context in which it appears.
In order to record a color image in a single exposure, a two-dimensional CCD array with rows and columns of CCD elements is typically overlaid by a color filter mosaic, such as a RGB filter mosaic, a CMY filter mosaic, etc, whereby each light sensitive CCD element (pixel) in the CCD-array is covered with a filter that allows passage of light within a specific wavelength range corresponding to a specific color. Since each pixel is illuminated by one specific color, resolution of the recorded color image is lower than the original grey tone resolution of the CCD array since each image pixel corresponds to a plurality of CCD elements. Thus, a color filter mosaic allows a color image to be recorded in a single exposure at the expense of resolution.
It is well known in the art to improve spatial resolution of a color image by estimation of a color that is not recorded by a specific CCD element by interpolation. Numerous interpolation algorithms and interpolation strategies are known in the art. For example, U.S. Pat. No. 5,373,233 discloses a color interpolation method.
It is well known that digital images of objects having some repetitive regular structures may be distorted by Moiré effects.
The Moiré effect is created by interference between two or more regular structures, e.g. with different spatial frequencies as shown in FIG. 2. Two linear patterns 1, 2 with different spatial frequencies are shown generating frequency beating. Likewise, a Moiré effect may be generated when two patterns are rotated or displaced with respect to each other as shown in FIG. 3. Similarly, colored patterns may create colored Moiré patterns which is termed color Moiré throughout the present disclosure.
When recording images using an image recording system that images an object onto a two-dimensional CCD array, a first pattern is inherent in the recording system namely the pattern constituted by the matrix of CCD elements. Therefore, the recording of a digital image of any object having some repetitive structure may produce an image with Moiré patterns.
Various methods for reducing Moiré effects are known:
1. Using a quasi-periodic patterned CCD, in which the CCD elements do not form a regular matrix. Instead they form an irregular pattern of elements with varying distances between them. The irregular matrix has no definite spatial frequency, and hence interference with a spatial frequency in the image is prevented (U.S. Pat. No. 4,894,726).
2. Optical blurring for distorting an object pattern in the formed image, e.g. by inserting an anti-aliasing filter in front of the image.
The idea of aggravating image resolution in order to corrupt the spatial frequency producing Moiré effects is also utilized in the next two methods.
3. Optical low-pass filters may cause different colors of incident light to propagate along different paths. Thereby any patterning in the formed image are “color-separated”, hence the color resolution is reduced thereby reducing Moiré effects (U.S. Pat. No. 5,237,446).
4. By optical methods disclosed in U.S. Pat. No. 4,607,287 or U.S. Pat. No. 4,998,164, the spatial frequency of the recording device is corrupted by swinging or wobbling the CCD array.
It is a disadvantage of method 2, 3 and 4 for suppression of Moiré effects that the recorded image resolution or sharpness is decreased.
It is a further disadvantage of method 2 and 3 that an extra optical element is inserted in the beam path to corrupt or filter the spatial frequency in the image causing increased complexity and cost and also attenuation of recorded light intensity.
It is a disadvantage of method 1 that cost expensive non-standard CCD arrays comprising irregular shaped sensor elements are required.
It is a disadvantage of method 4 that the displacement is one-dimensional so that only one of the two spatial frequencies of the CCD array are efficiently removed.